French patent No.2,424,733 describes an in-body probe constituted by a catheter forming a flexible sheath containing a flexible coupling cable connected at one of its ends to a support block on which at least one ultrasound transducer is mounted, which transducer is connected outside the catheter to a processing and control unit. The other end of the cable is secured to a rotary drive member for rotating the support block.
Such a probe can be used to determine speed by means of the Doppler effect, and if the diameter of a blood vessel is known by other means, it is thus possible to determine the flow rate of blood flowing within the vessel. It is recalled that flow rate is equal to the cross-section of the vessel multiplied by the mean speed of the velocity distribution inside the vessel.
Although the above-described probe has made considerable progress possible in the measurement of aorta flow rates, it nevertheless turns out that the flow rate measurements performed using such a probe are lacking in accuracy.
Such a probe measures the mean speed by use of an ultrasound transducer whose beam does not cover the entire area of the right cross-section of the vessel. In addition, in practice, it is difficult or even impossible to ensure that the probe is pointing in the right direction for the purpose of measuring flow rates. Wrong positioning of the probe can give rise to Doppler signals that have all of the appearances of the looked-for signals, but that give rise to inexact flow rate values. Because of wrong positioning, the ultrasound beam may also cover, in part, a zone lying outside the vessel, such that the measurement device takes moving elements into account that lie outside the vessel, and this constitutes a source of error.
It should also be observed that in the prior art device, the treatment performed on the Doppler signal is incapable of excluding movement of the walls of the vessel since all signals coming from moving targets situated anywhere along the ultrasound beam are taken into account.
Furthermore, during the heart beat cycle, and in particular during diastole, the velocity distribution profile includes zones of zero speed or of speeds that are so low as to be incapable of being measured by means of the probe and the processing device in combination. The mean velocity distribution measured in this way is therefore erroneous, since account is taken only of targets that are moving fast enough. Under such conditions, flow rate measurement can give rise to an error that is extremely large given that the measured mean speed is multiplied over the entire cross-section of the vessel. The greater the relative fraction of the total cross-section of the vessel that is occupied by liquid streams that are stationary or too slow to be detected by the probe, the greater the error.